This invention relates generally to a composite sensor which detects acceleration or angular velocity and in particular to a composite sensor that has been constructed by micro-machining silicon.
Some composite systems combining an acceleration sensor and an angular velocity sensor have been made public recently. One of the representative systems pertains to an automobile attitude control, commonly called VDC (vehicle dynamic control) or VSC (vehicle stability control). These composite sensor systems are used in the comprehensive control of the movement of automobiles by the adjustment of brake pressure of individual wheels for the purpose of preventing over or under steering at the time of rotational movement of the vehicle. For such a purpose, a two-axis acceleration sensor and a one axis angular velocity sensor are deemed to be essential.
Navigational systems of automobiles have quickly become popular in recent years. In such systems, the signal of wheel velocity, etc., as received from the automobile itself, is used in the calculation of the running distance of the automobile. However, the connection for signal reception is troublesome and as a result there is a demand for a stand alone system. Therefore, a navigation system has been proposed for accurately knowing the position of an automobile by detecting the change in direction of its progress at an intersection or a curve by means of an angular velocity sensor and by determining the running distance by means of an acceleration sensor, thereby accurately knowing the position of the automobile. An angular velocity sensor and an acceleration sensor which are used in a composite sensor according to the prior art will be explained briefly by referring to the attached drawings.
With reference to FIG. 9, reference numeral 102 indicates a conventional angular velocity sensor having a metallic body 103 in the shape of a rectangular parallelepiped. Piezoelectric bodies 104.sub.1 and 104.sub.2 are provided on metallic body 103 on two respective sides normal to one another.
Angular velocity sensor 102 is arranged so that a moving body with a rotary axis 105 forms the Z axis in the case where the normal lines of the vibration surface of the piezoelectric bodies 104hd 1 and 104.sub.2 are made along the X axis and Y axis respectively, as shown in FIG. 9(A).
In the case where piezoelectric body 104.sub.1 is set as the driving side and piezoelectric body 104.sub.2 is set as the detection side, if an alternating current voltage is impressed on piezoelectric body 104.sub.1 on the driving side, thereby vibrating it in the X axis direction as is shown in FIG. 9(B), the rotary movement of the moving object in which the sensor is mounted produces a Coriolis force, thereby generating a vibrating force in a direction (the Y axis direction) which crosses, at a right angle, with the direction of vibration of piezoelectric body 104.sub.1 on the driving side. Its vibration is transmitted to metallic body 103 and piezoelectric body 104.sub.2 on the detection side is vibrated.
The cycle of the vibration of piezoelectric body 104.sub.2 on the detection side that is generated at that time is the same as the cycle of the vibration in the X-axis direction of the piezoelectric body 104.sub.1 on the driving side, with the amplitude being of such magnitude as is in conformity with the Coriolis force. Since the electromotive force is generated in piezoelectric body 104.sub.2 as a result of the vibration of the piezoelectric body 104.sub.2 on the detection side, the Coriolis force can be detected by measuring its magnitude, thereby obtaining the magnitude of the angular velocity.
With reference to the acceleration sensor, in FIG. 10 the acceleration sensor 202 according to the prior art is composed of base 205 of Pyrex plate glass, an etched silicon mass plate 206 and a cap 207 of Pyrex plate glass having a concave recess in the bottom thereof. Base 205, mass plate 206 and cap 207 are tightly jointed together in this order.
A mass portion 211 having a relatively large thickness and a bridge portion 212 having a relatively thin thickness are provided within a recess of mass plate 206 so that mass portion 211 is movably supported inside acceleration sensor 202.
On the upper surface of mass portion 211 and on the facing bottom surface of the recess in cap 207, electrodes 204.sub.1 and 204.sub.2 made of a thin metal film are formed, respectively, resulting in a variable capacitor of the parallel plate-type formed by electrodes 204.sub.1 and 204.sub.2.
When sensor 202 is subjected to acceleration, mass portion 211 is displaced in conformity with the size of the acceleration in the normal direction of electrodes 204.sub.1 and 204.sub.2, with a result that the distance between the two electrodes 204.sub.1 and 204.sub.2 changes. As a consequence, the capacitance of the capacitor which is formed by electrodes 204.sub.1 and 204.sub.2 changes. If the amount of the change is detected, the magnitude of the acceleration can be measured.
However, the manufacturing steps for the angular velocity sensor and the manufacturing steps for the acceleration sensor as described above are completely different. Such separately manufactured items combined to make a composite sensor, are expensive, more cumbersome and larger in size than is desirable.
In addition, a composite sensor prepared by combining the acceleration sensor and angular velocity sensor as described could not conveniently be arranged on a printed substrate and would be difficult to connect to a measuring circuit. It is therefore desirable to provide a solution to this problem.